In a cellular system employing direct sequence code divisional multi access, since the same frequency bands are employed in a plurality of channels, other channels' radio signals become interference, and the reception quality of a desired wave deteriorates when interference increase, thereby causing disconnection of a link or the like. Therefore, the number of links through which talking at the same time is possible while a constant standard of speech quality is maintained, that is, a link capacity, depends on the amount of the interference.
Thus, in a cellular system employing the direct sequence code divisional multi access, when transmission from a base station to a plurality of mobile stations is implemented, the transmission power value of the base station is controlled in order that a downlink signal that each mobile station generally receives is at a required minimum reception level. That is, a closed loop control is implemented wherein while each mobile station periodically measures the reception quality of a signal that the base station transmits, a signal instructing the base station to decrease the transmission electric power is transmitted when the measured reception quality exceeds a predetermined target value, and a signal instructing the base station to increase the transmission electric power is transmitted when the quality is less than the predetermined target value. Typically, in this control, a high speed closed loop type control for each slot is performed so that the reception level in a mobile station becomes a minimum reception level capable of talking.
In general, in handover of a cellular system employing the direct sequence code divisional multi access, a technique called soft handover is employed. This technique is one in which when a mobile station reaches to a cell boundary area of a base station with which the mobile station communicates so that the reception level of downlink decreases due to a propagation loss or the like, a link is set up also with a base station adjacent to the base station to implement simultaneous connections. Thus, the mobile station periodically measures pilot signals that each base station transmits at a predetermined electric power, and when a base station exists in which the difference between the reception level from a presently communicating base station and that thereof becomes within a predetermined threshold value, base stations are chosen so that the number of the base stations is within a predetermined maximum number in the order of good quality to implement simultaneous connections.
In a mobile station during the soft handover, the same down information is received from a plurality of base stations to implement a diversity synthesis. In a vicinity of a cell boundary, since the distance from a base station is long, the reception quality deteriorates. Although link quality tends to deteriorate due to interference from another cell, by a diversity effect caused by connection with such plurality of base stations, deterioration in downlink quality can be prevented.
However, since a plurality of links are employed for one mobile station in the soft handover, interference in downlink increases compared to a cellular system in which the soft handover is not employed, causing a problem that increase in the link capacity cannot be achieved.
As a technique for solving this problem, Japanese Patent Publication No. H11(1999)-69416 discloses a transmission electric power control technique of a base station selection type in which the downlink capacity is increased by specifying transmission base stations actually transmitting among a base station group during the soft handover.
In this transmission electric power control technique of the base station selection type, a base station group and links are set up wherein the differences in the reception qualities are within a threshold value, and further transmission base stations to actually transmit are selected from among the base station group with which links are set up. The transmission base station is supposed to be one in which the difference in the reception qualities is within an inner threshold value which is smaller than the threshold value of the time of setting links up, and the base station other than the transmission base station with which links are set up switches its transmission electric power to a predetermined minimum transmission power value. Thus, it becomes possible to reduce interference of downlink while maintaining an advantage in smooth handover switching in the conventional soft handover, and therefore the link capacity can be increased.
However, since there is a delay time, for transmitting a measured result to the base station, between time of measuring the reception quality and time of actually transmitting, and since the reception level is changing due to fading or the like, the reception level changes during the delay time, and a drift occurs between the measured result and the reception quality of the time of actual transmission.
Accordingly, even when a transmission base station is decided in the order of good quality at the time of measuring the reception quality, a case where a base station having a good quality is not included in the transmission base stations at the time of actual transmission may occur. In this case, not the best base station but a base station having a greater propagation loss has to transmit. Typically, since the reception quality at a mobile station is controlled so as to satisfy a desired standard by a transmission electric power control of a high speed closed loop type, there is a problem that the transmission electric power of a base station is high, interference electric power for other mobile stations increases, and the link capacity decreases.
Accordingly, it is an object of the present invention to decrease the number of transmission base stations and increase the downlink capacity while enhancing the possibility that the best base station is included.